Browning of Fruit
Browning of apples and other fruit from damage, such as cuts, bruises, slicing, juicing, cell death, or any other form of damage that disrupts cell membranes, is believed to be caused by the enzymatic reaction catalyzed by Polyphenol Oxidase (PPO). The brown pigment is a polymer formed from the non-enzymatic condensation of quinones, with lesser amounts of amino acids and proteins, into lignin-like compounds. The quinones are synthesized from di-phenols in the enzymatic reaction catalyzed by PPO (Whitaker and Lee, 1995). Most PPOs also have monophenolase activity and convert monophenols to di-phenols (Mar-Sojo et al., 1998). The cause of browning has been understood for some time, yet the solution to reducing browning remains an on-going problem for industry.
Browning reduces quality by causing detrimental flavor and nutritional changes (Eskin, 1990). With the explosive growth of the fresh-cut produce sector, lost opportunities have become more evident given that, for instance, browning limits the use of apple and other fruits in commercial fresh cut produce products. It is thus a significant problem limiting the widespread introduction of fresh cut produce products such as prepared apple slices.
Browning is also a major consideration in the manufacture of juice, and brown bruises are a significant cause of reduced grade for growers and of lost value for institutional processors (restaurants, hospitals, etc.) and retailers, who have to accept these losses or try to minimize them through the implementation of improved handling practices.
Approaches to Control Browning
Various approaches to control vegetable and fruit browning have been described and resulted in mixed success, due to a variety of reasons, including cost and amount of handling. For a general review of strategies for reducing fruit browning, see e.g.: Friedman (1991), Iyengar and McEvily, (1992), Whitaker and Lee (1995), McEvily et al. (1992), Sapers (1993), Weemaes (1998), Martinez and Whitaker (1995), and Brushett (2006).
U.S. Pat. No. 5,939,117 (Cheng et al.) and U.S. Pat. No. 5,925,395 (Cheng) describes an anti-browning/anti-oxidant dip treatment. Fresh-cut apple slices which have been treated with an anti-browning/anti-oxidant dip, are described as having reduced browning. However, the off-flavoring and high cost of the anti-browning/anti-oxidant dip solution has limited their commercial success. Furthermore, anti-oxidant dip solutions do not deal effectively with secondary browning that results from the cutting knife and skin deformation prior to cutting and other secondary browning reactions, which lead to a thin brown line under the skin on the apple slice and other market detracting attributes.
Other approaches to control browning have been described, including but not limited to, cultivation in low oxygen atmosphere and low temperature (Heimdal et al., 1995); treatment with calcium ascorbate, glutathione, cysteine and citrate (Jiang et al., 1998); treatment with sulfites and sub optimal pH and high-pressure carbon dioxide (Chen et al., 1992); treatment of fresh cut apple slices with natural products (Buta et al., 1999); and a treatment with a 10% solution of honey (Osmianski and Lee, 1990).
Murata et al. (2000 and 2001) report that by suppression of a single PPO gene homologous to the apple PPO gene APO5 they obtained apple shoots and callus having reduced PPO activity which exhibit low browning potential in vitro. However, these references do not disclose a reduced browning fruit-producing plant or reduced-browning apple nor whether suppression of a single PPO gene homologous to APO5 would be sufficient to obtain such a reduced-browning fruit-producing plant or reduced-browning apple.